High-CO2 air from over land often has the concentration drop more realistic estimate of how much drop can be caused by the

oceans (over large areas) is obtained by comparing measurements of CO2 at Cape Grim Tasmania which, when measured in air coming off the ocean averaged about 1 ppm lower

than air measured by CSIRO on flights over Bass Strait.

93. p. 419: The lowest figure measured since 1812, the 270 ppm figure, is taken as the pre-industrialisation yardstick. The IPCC want it both ways. They are prepared to use the lowest determination by the Pettenkofer method as a yardstick yet do not acknowledge P e t t e n k o f e r m e t h o d m e a s u r e m e n t s s h o w i n g C O 2 c o n c e n t r a t i o n s f a r h i g h e r t h a n n o w m a n times since 1812. The IPCC does not use 270 ppm as the pre-industrial CO2 concentra- tion. The value used is 280 ppm. In the various WG1 reports, see SPM table 1 in 1990, technical summary (TS) table 1 in S R, TS table 1 in T R, and page 2 in SPM of R4. This number is based in measurements of air in ice bubbles (mainly using IR techniques) and excluding anomalously low values from the time of the Little Ice ge. For ice cores, the volume of air available is too small to use the less precise chemical (Pettenkofer) method. y